1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a liquid crystal display with a patterned common electrode.
2. Description of Related Art
A thin film transistor liquid crystal display (hereinafter referred to as “TFT LCD”) is being widely used because it has a number of beneficial characteristics including a large contrast ratio, suitability for realizing gray scale or moving pictures, and a capability to easily realize full colors.
As the display size of the TFT LCD and its resolution increase, however, deterioration of images due to such phenomena as crosstalk, flicker and residual image are becoming increasingly problematic. The crosstalk refers to the phenomenon through which a gray level different from a predetermined gray level is displayed at peripheral regions positioned above and below or left and right of a region A when white or black is displayed at the region A. It is because the peripheral regions are influenced by the white or black of the region A. A vertical crosstalk is the crosstalk generated at the peripheral regions positioned above and below the region A, and a horizontal crosstalk is the crosstalk generated at the peripheral regions positioned to the left and right of the region A.
FIG. 1 is a plan view of a screen of a conventional normally white type liquid crystal display on which horizontal crosstalk has been generated. In FIG. 1, a screen having windows W11 to W33 arranged in a matrix configuration is illustrated.
To display “black” on the window W22 and “gray” on peripheral windows W11 to W13, W21, W23 and W31 to W33, first the scan lines of the windows W11, W12 and W13 are selected, and a first voltage is equally applied to the data lines of the windows W11, W12 and W13. Then, the scan lines of the windows W21, W22 and W23 are selected, and the first voltage is applied to the data lines of the windows W21 and W23 while a second voltage higher than the first voltage is applied to the data line of the window 22. Finally, the scan lines of the windows W31, W32 and W33 are selected and the first voltage is equally applied to the data lines of the windows W31, W32 and W33. On the other hand, a substantially uniform voltage is applied to the common electrode for the entire screen.
The voltages of the data lines are swung periodically during the selection time of the scan lines to prevent deterioration of the liquid crystal. Consequently, a coupling capacitor is produced due to capacitive coupling between the data lines and the common electrode, and the voltage of the common electrode is distorted. Further, the higher the voltage applied to the data line, the wider the voltage swing is, and the distortion is increased accordingly.
The distortions of the voltage of the common electrode of the windows W11, W12 and W13 are substantially the same so that crosstalk is not generated since the first voltage is equally applied to the data lines of the windows W11, W12 and W13 when the scan lines of the window W11, W12 and W13 are selected. In the same manner, the crosstalk is not generated for the windows W31, W32 and W33. However, on the windows W21, W22 and W23, the distortion of the voltage of the common electrode of the window W22 is greater than the distortion of the windows W21 and W23 since the second voltage applied to the data line of the window W22 is greater than the first voltage applied to the windows W21 and W23. The greatly distorted voltage of the common electrode of the window W22 influences the voltage of the common electrodes of the windows W21 and W23, so that the gray level of the windows W21 and W23 is changed. Therefore, brighter gray than the gray originally intended to be displayed is displayed on the windows W21 and W23. This phenomenon is called “horizontal crosstalk”.
When the common electrode is positioned over the entire upper substrate, a coupling capacitor between the data lines and the common electrode is easily produced and the distortion of the common electrode voltage of the window W22 easily influences the common electrode voltage of the windows W21 and W23. Therefore a horizontal crosstalk is easily produced.
A column inversion driving method has been adopted to solve the horizontal crosstalk problem. However, the column inversion driving method increases the level of voltage applied to the TFT so that power consumption is increased.